Are there 64-bit and 32-bit DLL versions?

In short no, GemBox components work on both 32-bit and 64-bit machines.

GemBox.Email is fully managed .NET library which is independent of the system architecture. If application which is using GemBox.Email targets 64-bit then GemBox.Email will also use 64-bit architecture, since it is JIT compiled when application runs, and if it targets 32-bit GemBox.Email will use 32-bit architecture instead.